What exactly is "down" about it? The wiki page itself? Or a page linked there?
The links on the documents pages (Zimbra Documentation Links - Zimbra :: Wiki or Documentation for the Zimbra Collaboration Suite; open source email, contacts, and group calendaring ) reference many links for the html version of the docs. When a link is selected, a blank page is loaded. Hopefully it is not just me.
It's a known issue and is being worked on.
There are pdfs and google caches available in the meantime.